The present invention relates to a two-beam pickup type optical disk drive for irradiating two light beams to an optical disk, recording or reproducing information using one of the light beams, and deleting information using the other light beam.
In the past, a one-beam pickup type optical disk drive have been employed in irradiating one light beam (for example, laser light) to an optical disk and thus recording, reproducing, or deleting information. In general, the one-beam pickup type optical disk drive uses a voice coil motor (will be abbreviated to a VCM) and a lens actuator (incorporated in an optical head) to move the light beam to a track in which information is recorded. The voice coil motor drives the optical head that is an output source of a light beam, and the lens actuator drives a condenser, which converges the light beam, in a radial direction of the optical disk. The one-beam pickup type optical disk drive is disclosed in, for example, Japanese Unexamined Patent Publication (JP-A) No. 61-177641.
The one-beam pickup type optical disk drive will be briefed below. The optical disk drive has a velocity profile representing a pattern of moving velocities that should be assumed by a light beam traveling from any position to a target track. In other words, the velocity profile represents the variation of a function of time required to enable the light beam to reach the target track for the shortest time. The velocity profile is pre-set when the target track is determined.
For enabling a light beam to reach a target track, a positional deviation of the light beam from the target track is detected. The VCM is used to control the position of an optical head so that the positional deviation will become nil. A lens actuator is used to give control so that the pattern of moving velocities assumed by the light beam until the light beam reaches the target track will agree with a velocity profile. The moving velocity of the light beam is calculated by counting the number of tracks traversed by the light beam within a certain time (hereinafter, the number of traversed tracks).
On the other hand, a two-beam pickup type optical disk drive has been disclosed in, for example, Japanese Unexamined Patent Publication (JP-A) No. 3-88129. The two-beam pickup type optical disk drive irradiates two light beams to an optical disk, records or reproduces information using one of the light beams, and deletes information using the other light beam.
However, in the two-beam pickup type optical disk drive of the prior art disclosed in the Japanese Unexamined Patent Publication No. 3-88129. The actions for bringing the second light beam to a tracking state are unstable. It therefore takes much time for the first light beam and second light beam to enter the tracking state. This poses a problem in that it takes much time to access a target time.
The present applicant has proposed a two-beam pickup type optical disk drive in an effort to solve the foregoing problems. The disk drive is disclosed in Japanese Unexamined patent Publication (JP-A) No. 9-204670.
In the optical disk drive described in the patent publication, a first tracking control unit calculates the moving velocity of the first light beam from the number of tracks traversed by the first light beam. The velocity is controlled so that the velocity varies in conformity to a predetermined velocity profile. The position of an optical head is controlled so that the optical head follows the first light beam.
Moreover, when the first light beam is accelerating or traveling at a constant velocity, a second tracking control unit controls the position of a second light beam so that the second light beam follows the optical head. Furthermore, the number of remaining tracks ending with a target track of the second light beam is calculated from the number of tracks traversed by the second light beam. While the first light beam is decelerating, the moving velocity of the second light beam is calculated from the number of tracks traversed by the second light beam. The velocity of the second light beam is controlled so that the velocity varies in conformity to the predetermined velocity profile.
However, a problem underlies the optical disk drive described in the Japanese Unexamined Patent Publication No. 9-204670. Specifically, assuming that the first light beam is accelerating or traveling at a constant velocity, a large error may occur in the count of tracks traversed by the first light beam or second light beam. In this case, the position of the first light beam having reached a target track and the position of the second light beam having reached it are greatly deviated from each other in a radial direction. Consequently, the actions for bringing the light beams to a tracking state get unstable.
Factors causing the large error in the count of tracks traversed by the first light beam or second light beam include a defect in an optical disk, a flaw thereon, and deterioration in quality of a tracking error signal. Consequently, the count of tracks may indicate a smaller number of tracks than the number of actually traversed tracks.
Moreover, even when the count of tracks traversed by the second light beam indicates a larger number of tracks than the number of actually traversed tracks, the actions for bringing the second light beam to the tracking state get unstable.